MP3s are, by definition, lossy, meaning the sound was changed from the original in order to make it fit into a small amount of space. The changes are made in ways that are meant to fool the ear, but if there's not enough space, sacrifices have to be made. There are also just some sacrifices that are made as part of the MP3 conversion process, changes which occur regardless of the available space. Sometimes the various changes made to the audio can be heard as audible artifacts/noise. There's really no practical way to "clean up" this audio and remove the noise; if there were, the MP3 encoder would've done so. Anyone who says they have a "noise reducer for MP3s" is selling snake oil.

Well there are photoshop plugins that reduce artefacts from jpg compression, why it wouldn't be possible for mp3 compression ? It's not about reconstituting original signal, but making artefacts less obvious.

What kind of noise are you trying to remove? What kind of recording is this? What's the story? Where did the noise come from?

If you have a slight background noise, it might be possible to remove or reduce it. But in cases where the noise is bad, it is usually impossible to remove the noise without damaging the "good" audio. For example, you can often remove background "hiss" or "hum", but if you have a conversation recorded at a party, or if there is a dog barking during a conversation, there is little hope.

Just about every audio editor (including Audacity which is FREE) has a variety of filters and noise reduction tools. There is a common noise reduction tool/method where you feed-in a "noise fingerprint", and the software tries to remove the noise from the good sound.

The only catch is, when you open the file for editing/processing with any "normal" audio editor, it gets de-compressed from MP3. That means you have to go through a 2nd lossy compression step if you want to re-save as MP3 (or other lossy format). You may not notice any additional quality loss, especially if you use a high-quality MP3 setting. But, it's something to be aware of.

Well there are photoshop plugins that reduce artefacts from jpg compression, why it wouldn't be possible for mp3 compression ? It's not about reconstituting original signal, but making artefacts less obvious.

Well there are photoshop plugins that reduce artefacts from jpg compression, why it wouldn't be possible for mp3 compression ?

These are usually some variation on low pass filtering. You can low pass filter audio if you want to. It doesn't help very much though.

You can do much better than (straight) lowpass filtering with JPEG. The main issue is visible blocking artifacts around the 8x8 pixel non-overlapping DCT boundaries. If the quantizer steps are very large and the gradient across block boundaries are sufficiently large, chances are that the image will look better by decreasing that gradient (but not all gradients, like a lowpass filter would).

mp3 uses MDCT so such blocking artifacts should not be present. I do not know enough about mp3 to make any claims about possible other sub-optimal characteristics that might be "prettyfied" post decoder.

What about blind high-frequency reconstruction based on lowfrequency bands wherever encoder lowpass/quantizer have removed all energy?

Reveals the delicate and aesthetically delightful details in the music

Restores warmth to the spoiled digitally compressed sound

Expands the upper frequency range by over-sampling

QUOTE (knutinh @ Jun 29 2012, 03:50)

What about blind high-frequency reconstruction based on lowfrequency bands wherever encoder lowpass/quantizer have removed all energy?

I'm guessing that's what "Restores and enhances harmonics lost through compression" is, with a very generous use of the word "restores", given that there's no way to know exactly what was lost. Something SBR-like, maybe? Regardless, if it has the effect of boosting the higher frequencies and adding some noise up there (judiciously or not), I'm sure people will enjoy it and say it sounds better than a typical 16 KHz-lowpassed MP3, thus "restored". I saw on another forum it was described as "polishing a turd".

I'm guessing that's what "Restores and enhances harmonics lost through compression" is, with a very generous use of the word "restores", given that there's no way to know exactly what was lost. Something SBR-like, maybe? Regardless, if it has the effect of boosting the higher frequencies and adding some noise up there (judiciously or not), I'm sure people will enjoy it and say it sounds better than a typical 16 KHz-lowpassed MP3, thus "restored". I saw on another forum it was described as "polishing a turd".